Addition of Monovalent Salts for Improved Viscosity of Polymer Solutions Used in Oil Recovery Applications

ABSTRACT

An oil recovery composition having a brine, a hydrolyzable polymer, and a monovalent cations to divalent cations ration in the range of about 2.5:1 to 3:1 is provided. An oil recovery composition may be formed from a brine recovered from production water and a polymer. A monovalent salt may be added to the brine recovered from production water to form a modified brine and achieve a modified monovalent cations to divalent cations ratio in the range of about 2.5:1 to 3:1. Processes for forming the oil recovery composition and enhanced oil recovery using the oil recovery composition are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority from U.S.Non-provisional application Ser. No. 15/660,420 filed Jul. 26, 2017, andtitled “ADDITION OF MONOVALENT SALTS FOR IMPROVED VISCOSITY OF POLYMERSOLUTIONS USED IN OIL RECOVERY APPLICATIONS,” which claims priority fromU.S. Provisional Application No. 62/366,926 filed Jul. 26, 2016, andtitled “ADDITION OF MONOVALENT SALTS FOR IMPROVED VISCOSITY OF POLYMERSOLUTIONS USED IN OIL RECOVERY APPLICATIONS,” each of which areincorporated by reference in their entirety for purposes of UnitedStates patent practice.

BACKGROUND Field of the Disclosure

Embodiments of the disclosure generally relate to an oil recoveryprocess and enhancing oil recovery from a reservoir formation. Inparticular, embodiments of the disclosure relate to an oil recoveryprocess using a polymer.

Description of the Related Art

The use of improved oil recovery (also referred to as enhanced oilrecovery (EOR)) processes has greatly benefited the oil and gas industryby increasing the production of hydrocarbon bearing wells and fields.The EOR processes used in modern oil and gas operations may includechemical, gas, thermal, and microbial based processes. Water injection(alternatively referred to as water flooding) has been widely used tomaintain reservoir pressures and displace hydrocarbon toward wells, thusincreasing the production of liquid hydrocarbons in subterraneanreservoir. Chemical EOR applications are water-based and use chemicalssuch as polymers, surfactants, alkalines, or combinations thereofdissolved in water and co-injected. The water source may be derived fromfreshwater, (for example, aquifers or surface water), saltwater/brackishsources on the surface (for example, river/sea water mixtures), or inwater reservoirs (for example, aquifer water or brines coproduced fromoilfield reservoirs).

SUMMARY

Water flooding, including the injection of water into a reservoirformation, may be used in EOR processes. Some EOR techniques add apolymer to the water to increase the viscosity of injected water toachieve more favorable mobility and counteract heterogeneity effects.Depending on the source water and its composition, more resistantpolymers, higher concentrations of polymers, or both may be used to forman EOR solution having a desired viscosity. However, the use of suchpolymers at the relatively high concentrations needed to achieve adesired viscosity may increase operating costs of EOR operations andconsume large quantities of the polymer. The salinities and hardness ofthe water used in such EOR operations may result in lower viscosities ofpolymer solutions and may require the use of more resistant polymers andrelatively high concentration of polymers. As an alternative, watersoftening facilities may be used to enable the use of less resistantpolymer or lower concentrations of polymer, but the use of suchfacilities may increase the capital cost of EOR operations.

In some embodiments, a composition for enhancing oil recovery in ahydrocarbon reservoir formation is provided. The composition includes amodified brine having a modified monovalent cations to divalent cationsratio in a range of 3:1 to 4.5:1 and a hydrolyzable polymer having aconcentration of 0.05 weight (wt) % to 0.5 wt %. The monovalent cationsto divalent cations ratio is selected to achieve a target viscosity ofthe composition, and the modified brine formed by adding a monovalentsalt to a brine to modify the ratio of monovalent cations to divalentcations of the brine. In some embodiments, the hydrolyzable polymerincludes polyacrylamide. In some embodiments, the monovalent cationsinclude sodium cations. In some embodiments, the divalent cationsinclude at least of calcium cations and magnesium cations. In someembodiments, the target viscosity is at least 4 centipoise at conditionsof the hydrocarbon reservoir formation. In some embodiments, the brineincludes brine recovered from production water. In some embodiments, thebrine includes treated seawater or untreated seawater. In someembodiments, the brine includes aquifer water. In some embodiments, themonovalent salt includes sodium chloride (NaCl). In some embodiments,the composition includes a surfactant.

In some embodiments, a method of enhancing oil recovery in a hydrocarbonreservoir formation is provided. The method includes introducing an oilrecovery composition into the reservoir formation. The oil recoverycomposition includes a modified brine having a modified monovalentcations to divalent cations ratio in a range of 3:1 to 4.5:1 and ahydrolyzable polymer having a concentration of 0.05 weight (wt) % to 0.5wt %. The monovalent cations to divalent cations ratio is selected toachieve a target viscosity of the composition, and the modified brineformed by adding a monovalent salt to a brine to modify the ratio ofmonovalent cations to divalent cations of the brine. In someembodiments, the hydrolyzable polymer includes polyacrylamide. In someembodiments, the monovalent cations include sodium cations. In someembodiments, the divalent cations include at least of calcium cationsand magnesium cations. In some embodiments, the target viscosity is atleast 4 centipoise at conditions of the hydrocarbon reservoir formation.In some embodiments, the method includes preparing the oil recoverycomposition before introducing the oil recovery composition into thereservoir formation. The preparing includes recovering the brine fromproduction water, adding the monovalent salt to the brine to form themodified brine, and adding the hydrolyzable polymer to the brine. Insome embodiments, the brine includes treated seawater or untreatedseawater. In some embodiments, the brine includes aquifer water. In someembodiments, the oil recovery composition includes a surfactant.

In some embodiments, a method of forming a composition having a targetviscosity for enhancing oil recovery in a hydrocarbon reservoirformation is provided. The method includes determining a ratio ofmonovalent cations to divalent cations of a brine and adding amonovalent salt to the brine to modify the ratio of monovalent cationsto divalent cations of the brine to a range of 3:1 to 4.5:1. The methodalso includes adding a hydrolyzable polymer to the brine to form thecomposition having the target viscosity, wherein the hydrolyzablepolymer has a concentration of 0.05 weight (wt) % to 0.5 wt %. In someembodiments, the method includes recovering the brine from a productionwater. In some embodiments, the brine includes treated seawater oruntreated seawater. In some embodiments, the brine includes aquiferwater. In some embodiments, the method includes adding a surfactant tothe bring to form the composition. In some embodiments, hydrolyzablepolymer includes polyacrylamide. In some embodiments, the monovalentcations include sodium cations. In some embodiments, the divalentcations include at least of calcium cations and magnesium cations. Insome embodiments, the target viscosity is at least 4 centipoise atconditions of the hydrocarbon reservoir formation. In some embodiments,adding a monovalent salt to the brine includes mixing the monovalentsalt and the brine for a mixing time period. In some embodiments, themonovalent salt includes NaCl.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood with regard to the followingdescriptions, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of thedisclosure and are therefore not to be considered limiting of thedisclosure's scope as it can admit to other equally effectiveembodiments.

FIG. 1 is a plot depicting the viscosity of polymer solutions havingdivalent salts MgCl₂ and CaCl₂)) and a mixture of divalent salts andmonovalent salts (NaCl+MgCl₂ and NaCl+CaCl₂)) and a 0.25 percent byweight (wt %) of a high molecular weight (for example, 12×10⁶ Daltons)polyacrylamide at 95 degrees Celsius (° C.) in accordance with anembodiment of the disclosure;

FIG. 2 is a plot of the viscosity vs. rotational speed for a solution ofproduction water brine, 0.25 wt % polymer, and various NaClconcentrations in accordance with an embodiment of the disclosure;

FIG. 3 is a plot of the viscosity of a hypothetical oil recoverycomposition having various polymer concentrations and the addition of amonovalent salt in accordance with an embodiment of the disclosure; and

FIG. 4 is a flowchart of a process for forming an enhanced oil recoverycomposition from a brine recovered from produced water and having amonovalent cations to divalent cations ratio in the range of 3:1 to4.5:1.

DETAILED DESCRIPTION

The present disclosure will now be described more fully with referenceto the accompanying drawings, which illustrate embodiments of thedisclosure. This disclosure may, however, be embodied in many differentforms and should not be construed as limited to the illustratedembodiments set forth in the disclosure. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the disclosure to those skilled in the art.

Salts in the source brine used to form the polymer solution may haveadverse effects on polymer viscosity. Such salts may neutralizeelectrical charges along the polymer, enabling the polymer chain to coilinto a tightly wound sphere and reducing the surface exposed to thesolvent. This activity decreases interactions between the polymerparticles dissolved in the brine, decreases repulsion between thepolymer particles, and decreases the viscosity of the hydrolyzed polymersolution. Divalent salts may have a greater effect on polymer viscosityreduction than monovalent salts. The divalent cations present indivalent salts may suppress the viscoelastic behavior of the polymersolution at a greater amount than monovalent cations and minimizerepulsive forces among the charged groups present in the polymer chain.The hydrolyzed polyacrylamide interacts strongly with divalent cations,such as Ca′ and Mg′, which results in a reduction in moleculardimensions and, consequently, viscosity. Accordingly, the coilconformation of hydrolyzed polyacrylamide (HPAM) molecules in aqueoussolutions makes the molecules sensitive to the ionic environment.

For example, when comparing the viscosity values of divalent salts (forexample, MgCl₂ and CaCl₂)) and mixtures of divalent salts in thepresence of sodium chloride (NaCl+MgCl₂ and NaCl+CaCl₂)), the influenceof the monovalent ions clearly shows a reduction in the repulsion effectthat results in higher viscosities. FIG. 1 is a plot 100 depicting theviscosity of polymer solutions having divalent salts (for example, MgCl₂and CaCl₂)) and a mixture of divalent salts and monovalent salts(NaCl+MgCl₂ and NaCl+CaCl₂)) for polymer solutions and having a 0.25weight (wt) % of a high molecular weight polyacrylamide at 95° C. Asshown in FIG. 1, the Y-axis 102 corresponds to the viscosity incentipoise (cP) and the X-axis 104 corresponds to the dilution ratio ofthe salts. As shown in FIG. 1, the NaCl+MgCl₂ solution depicted by line106 has higher viscosities at all dilution ratios as compared to theMgCl₂ solution depicted by line 108. Similarly, the NaCl+CaCl₂) solutiondepicted by line 110 has higher viscosities at most dilution ratios lessthan about 14 as compared to the NaCl+CaCl₂) solution depicted by line112. Thus, it is believed that the monovalent cations may replace thedivalent cations at the polar sites along the polyacrylamide coils,permitting the coils to unwind and exert their viscous properties in thesolution when compared to the interactions of divalent cations at thesame sites.

In view of the foregoing, embodiments of the disclosure include oilrecovery compositions and processes for enhancing oil recovery from areservoir formation. The oil recovery compositions and processesdescribed in this disclosure may enhance oil recovery from a reservoirformation at reduced cost as compared to conventional enhanced oilrecovery compositions. In some embodiments, an oil recovery compositionincludes an oil recovery polymer and an aqueous solution having amonovalent cations to divalent cations ratio in the range of about 3:1to about 4.5:1. In some embodiments, the hydrolyzable polymer includespolyacrylamide. In some embodiments the hydrolyzable polymer includes apartially hydrolyzed polyacrylamide (HPAM). For example, in someembodiments, a suitable HPAM may have a molecular weight of about 12million, a degree of hydrolysis (HD) of 3.28%, and a solid content of90.56%. In some embodiments, the polymer is CA8036 manufactured byBomochem of China. In some embodiments, the aqueous solution includesmonovalent salts and divalent salts having a ratio in the range of about3:1 to about 4.5:1. In some embodiments, the aqueous solution is formedfrom a brine recovered from production water. As used in the disclosure,the term “production water” (also referred to as “produced water”)refers to water produced during the recovery of hydrocarbons from ahydrocarbon reservoir formation. In some embodiments, the aqueoussolution is formed from a brine of treated seawater or untreatedseawater. As used herein, the term “treated seawater” refers to seawaterthat is treated to remove components unsuitable for use in oil recoverycompositions, such as seawater that is treated to remove biofoulants,dissolved oxygen, suspended or dissolved solids, or any combinationthereof. In some embodiments, the aqueous solution is formed from abrine of aquifer water. In some embodiments, the aqueous solution may beformed by adding a monovalent salt to a brine having an initialmonovalent cations to divalent cations ratio such that an aqueoussolution (that is, a modified brine) having a monovalent cations todivalent cations ratio in the range of about 3:1 to about 4.5:1 isformed.

In some embodiments, a brine may be recovered from produced water andanalyzed to determine a ratio of monovalent cations to divalent cations.A monovalent salt may be added to the brine to modify the ratio ofmonovalent cations to divalent cations and achieve a target viscosity ofan oil recovery composition. The target viscosity may be determined forconditions of a hydrocarbon reservoir formation. In some embodiments,the ratio of monovalent cations to divalent cations may be modified tobe in the range of about 3:1 to about 4.5:1. In some embodiments, themonovalent salt is NaCl. In some embodiments the brine and monovalentsalts may be mixed for a mixing time period. An hydrolyzable polymer maybe added to the brine to form the oil recovery composition having thetarget viscosity and a monovalent cations to divalent cations ratio inthe range of about 3:1 to about 4.5:1. In some embodiments, the polymerhas a concentration of about 0.05% by weight to about 0.5% by weight. Insome embodiments, the hydrolyzable polymer is polyacrylamide.

In some embodiments, the oil recovery composition described in thedisclosure may be introduced (for example, pumped) downhole to enhanceoil recovery in a hydrocarbon reservoir formation. For example, an oilrecovery composition having a hydrolyzable polymer and a brine having amonovalent cations to divalent cations ratio in the range of about 3:1to about 4.5:1 may prepared at the surface and introduced downhole toenhance oil recovery from a hydrocarbon reservoir formation.

Examples

The following examples are included to demonstrate embodiments of thedisclosure. It should be appreciated by those of skill in the art thatthe techniques and compositions disclosed in the example which followsrepresents techniques and compositions discovered by the inventors tofunction well in the practice of the disclosure, and thus can beconsidered to constitute modes for its practice. However, those of skillin the art should, in light of the present disclosure, appreciate thatmany changes can be made in the specific embodiments which are disclosedand still obtain a like or a similar result without departing from thespirit and scope of the disclosure.

The following non-limiting examples of oil recovery compositions havingvarious monovalent cations to divalent cations ratios were tested andcompared against a solution of brine and a polyacrylamide polymer atvarious dilutions with water.

A first solution was formed from a brine recovered from production waterand a polyacrylamide polymer. The brine included a concentration ofmonovalent cations of 0.9638 milliequivalents per milliliter (meq/mL)and a concentration of divalent cations of 0.4926 meq/mL, such that themonovalent cations to divalent cations ratio of the first solution was1.96:1. The viscosity of the first solution was measured at variousdilutions with water.

A second solution was formed by adding a monovalent salt NaCl, to thefirst solution, such that the second solution has an increased ratio ofmonovalent cations to divalent cations. The viscosity of the secondsolution was measured at different concentrations of NaCl, that is atdifferent ratios of monovalent cations to divalent cations.

The polyacrylamide polymer used in the solutions is CA8036 partiallyhydrolyzed polyacrylamide (HPAM) manufactured by Bomochem of China. Thepolymer has a molecular weight of about 12×10⁶ Daltons. The degree ofhydrolysis is 3.28% and the solid content is 92.37%. The polymer wasprepared in concentrations of 0.25 wt % in the first solution and 0.25wt % in the second solution.

The dimensionless viscosity of each measured solutions was alsodetermined, according to Equation 1:

$\begin{matrix}{{\mu \; D} = \frac{{\mu \; x} - {\mu \; W}}{{\mu 6} - {\mu W}}} & (1)\end{matrix}$

Where μD is the dimensionless viscosity, μx is the viscosity at Xrevolutions per minute (RPM) in centipoise (cP), μw is the viscosity ofwater in cP (assumed to be 0.34), and μ6 is the viscosity at 6revolutions-per-minute (RPM) in centipoise (cP).

The viscosity of each solution was measured using a Brookfield DV-II+Proviscometer manufactured by Brookfield Engineering of Middleboro, Mass.,USA, using an S-18 Spindle sample adapter. The temperature wascontrolled using a TC-502 Temperature Control Bath by BrookfieldEngineering of Middleboro, Mass., USA. At high temperatures, a cap wasattached to the measuring cup to minimize evaporation. The measurementswere taken after a time period in order for the solution to reach thetemperature shown in the digital controller in the viscometer.

The composition of the brine recovered from production water and used inthe first and second solutions, in milligrams/liter (mg/L), is shown inTable 1:

TABLE 1 Composition of Example Brine Recovered from Production Water Na⁺22166 mg/L Ca²⁺  8128 mg/L Mg²⁺  1052 mg/L K⁺  1021 mg/L SO₄ ²⁻  384mg/L Cl⁻ 51810 mg/L CO₃ ²⁻   0 mg/L HCO₃  154 mg/L Total DissolvedSolids (TDS) 84175 mg/L Anions 52348 mg/L Cations 31346 mg/L MonovalentCations to 2.526 Divalent Cations Ratio (mass ratio)

The composition of the brine recovered from production water and used inthe first and second solutions, in milliequivalents/milliliter (meq/mL),is shown in Table 2:

TABLE 2 Composition of Example Brine Recovered from Production Water Na⁺0.9637 Ca²⁺ 0.4064 Mg²⁺ 0.0862 K⁺ 0.0262 SO₄ ²⁻ 0.0080 Cl⁻ 1.4594 CO₃ ²⁻0.0000 HCO₃ 0.0025 Anions (C₅₁) 1.4700 Divalent Cations (C₆₁) 0.4926Monovalent Cations to 2.01 Divalent Cations Ratio (meq/ml)

The composition of the second solution at different concentrations ofNaCl is shown in Table 3:

TABLE 3 Composition of Example Brine and Polymer Solution at DifferentNaCl Concentrations 2,500 mg/L 5,000 mg/L 10,000 mg/L 15,000 mg/L 21,300mg/L NaCl NaCl NaCl NaCl NaCl Na⁺ (mg/L) 23149 24133 26100 28067 30624Ca²⁺ (mg/L) 8128 8128 8128 8128 8128 Mg²⁺ (mg/L) 1052 1052 1052 10521052 K⁺ (mg/L) 1021 1021 1021 1021 1021 SO₄ ²⁻ (mg/L) 384 384 384 384384 Cl⁻ (mg/L) 53327 54843 57876 60909 64852 CO₃ ²⁻ (mg/L) 0 0 0 0 0HCO₃ (mg/L) 154 154 154 154 154 Anions (C₅₁) 1.513 1.555 1.641 1.7261.834 (meq/ml) Divalent Cations 0.493 0.493 0.493 0.493 0.493 (C₆₁)(meq/ml) Monovalent 2.096 2.183 2.356 2.530 2.749 Cations to DivalentCations Ratio (based on meq/ml) Monovalent 2.633 2.740 2.954 3.168 3.438Cations to Divalent Cations Ratio (mass ratio)

The viscosity of the first solution of production water brine and 0.25wt % polymer concentration was measured at various brine dilutions at95° C. The viscosity of the second solution of production water brine,0.25 wt % polymer concentration, and added NaCl was measured atdifferent NaCl concentrations at 95° C. Additionally, the dimensionlessviscosity for the solutions was determined from the measured viscosity.The viscosity and dimensionless viscosity at various speeds for thefirst solution, and the second solution having concentrations of NaCl,are shown in Table 4:

TABLE 4 Viscosity and Dimensionless Viscosity of Example Brine andPolymer Solution at Different NaCl Concentrations NaCl ConcentrationViscosity Speed Dimensionless (mg/L) μ (cP) ω (RPM) viscosity μ_(D)(unitless) Brine (no added 10.5 6 1.0 NaCl) 8.5 12 0.8 6.5 30 0.6 5.5 600.5 2,500 9.0 6 1.0 6.0 12 0.7 5.1 30 0.5 4.5 60 0.5 5,000 14.5 6 1.07.8 12 0.5 6.8 30 0.5 5.0 60 0.3 10,000 18 6 1.0 9.5 12 0.5 7.1 30 0.45.8 60 0.3 15,000 19.5 6 1.0 10.5 12 0.5 7.5 30 0.4 5.9 60 0.3 21,30012.5 6 1.0 7.5 12 0.6 5.8 30 0.4 4.8 60 0.4

FIG. 2 is a plot 200 of the measured viscosity vs. speed for the secondsolution of production water brine, 0.25 wt % polymer and different NaClconcentrations (that is, at different ratios of monovalent cations todivalent cations achieved by the addition of NaCl). As shown in FIG. 2,the Y-axis 202 corresponds to the viscosity in centipoise (cP) and theX-axis 204 corresponds to the speed of the viscometer in RPM. Theviscosities at the different concentrations of NaCl in the secondsolution are depicted as different lines in the plot 200. For example,the viscosities of the second solution at 2,500 mg/L NaCl are shown byline 206, the viscosities of the second solution at 5,000 mg/L NaCl areshown by line 208, the viscosities of the second solution at 10,000 mg/Lare shown by line 210, the viscosities of the second solution at 15,000mg/L NaCl are shown by line 212, and the viscosities of the secondsolution at 21,300 mg/L NaCl are shown by line 214. As shown in the plot200 and as mentioned supra, at higher concentrations such as 21,300mg/L, the viscosity of the second solution no longer increases ascompared to lower concentrations.

As shown in Table 4 and FIG. 2, the addition of a monovalent salt, NaCl,to the brine having both monovalent salts and divalent salts mayincrease the viscosity of the solution as greater concentrations of NaCland, consequently, increased ratios of monovalent cations to divalentcations are reached. At certain higher concentrations of monovalentsalt, such as 21,300 mg/L, the viscosity of the brine and polymersolution ceases to increase. As shown in Table 4, the increased NaClconcentrations increase the ratio of monovalent cations (such as Na⁺) inthe second solution to divalent cations (such as Ca²⁺ and Mg²⁺). Asdiscussed supra, a higher viscosity reduction may occur in the presenceof divalent cations. Thus, at these higher ratios of monovalent cationsto divalent cations, the viscosity reduction effect of the divalentcations discussed supra may be mitigated by the increased ratio ofmonovalent cations to divalent cations.

As also shown in Tables 4 and 5, the viscosity of the second solution atcertain NaCl concentrations is greater than the viscosity of the firstsolution at certain brine dilutions. As will be appreciated, theviscosity increases at lower speeds may be particularly applicable indownhole applications in a reservoir formation. Thus, a target viscosityfor an oil recovery composition may be achieved by adding a monovalentsalt to a desired monovalent cations to divalent cations ratio andwithout the addition of further polymer or water treatment. An oilrecovery composition at the target viscosity may have improvedresistance to high salinity or hardness and may be relatively lessexpensive as compared to other approaches used to achieve a targetviscosity, such as dilution, water softening, higher polymerconcentrations, more resistant polymers, or combinations of theseapproaches.

Accordingly, the use of the oil recovery composition described in thedisclosure may reduce the consumption of polymer and provide for the useof less resistant and less expensive polymer, thus lowering operationalcosts associated with the use of polymer oil recovery compositions.Additionally, use of the oil recovery composition described in thedisclosure may eliminate the use of water treatment (for example, watersoftening facilities) and further decrease the operational costs ofenhanced oil recovery operations.

Oil Recovery Compositions

In some embodiments, an oil recovery composition may include ahydrolyzable polymer and monovalent cations to divalent cations ratio inthe range of about 2.5:1 to about 3:1. In some embodiments, an oilrecovery composition may include a brine, a hydrolyzable polymer, and amonovalent cations to divalent cations ratio in the range of about 2.5:1to about 3:1. In some embodiments, the brine may be recovered fromproduction water. As used herein, the term “brine” may include syntheticseawater. In such embodiments, a monovalent cations to divalent cationsratio in the range of about 2.5:1 to about 3:1 may be achieved by addinga monovalent salt to the brine recovered from production water to form amodified brine having a modified monovalent cations to divalent cationsratio. In some embodiments, the brine is sourced from treated seawateror untreated seawater. In some embodiments, the brine is sourced fromaquifer water. In some embodiments, an oil recovery composition mayinclude a brine recovered from production water, polyacrylamide, and amonovalent cations to divalent cations ratio in a range of about 2.5:1to about 3:1. In some embodiments, an oil recovery composition may havea composition expressed as a ratio of monovalent salts to divalentsalts.

In some embodiments, the polymer may have a concentration of about 0.05wt % to about 0.5 wt %. For example, in some embodiments, thehydrolyzable polymer may have a concentration of about 0.25 wt %. Insome embodiments, the oil recovery composition may include a surfactant.

It should be appreciated that embodiments of the enhanced oil recoverycomposition (EOR) having a monovalent cations to divalent cations ratioin the range of about 3:1 to about 4.5:1 may include any suitablemonovalent salts and divalent. In some embodiments, the monovalent saltmay include NaCl. Other embodiments may include other monovalent saltsor divalent salts.

In some embodiments, the polymer of the oil recovery composition mayhave a maximum molecular weight that ensures the polymer retains theability to penetrate through a porous medium in a reservoir for whichthe oil recovery composition will be used. In some embodiments, thehydrolyzable polymer may include polyacrylamides, such as partiallyhydrolyzed polyacrylamides (HPAMs). In some embodiments, the polymer mayinclude a polyacrylamide having a molecular weight of 12×10⁶ Daltons.For example, in some embodiments, a suitable HPAM may have a molecularweight of about 12 million, a degree of hydrolysis (HD) of 3.28%, and asolid content of 90.56%. In some embodiments, the polymer is CA8036 HPAMmanufactured by Bomochem of China. In some embodiments, the hydrolyzablepolymer may include ionic polymers (for example, polymers having ionsacross the polymer chain) suitable for oil recovery applications.

In some embodiments, a target viscosity of an oil recovery compositionmay be achieved by the addition of a monovalent salt, such as NaCl, to abrine and polymer solution, such as an existing oil recoverycomposition. FIG. 3 is a plot 300 depicting the viscosity of ahypothetical oil recovery composition having various polymerconcentrations in accordance with an embodiment of the disclosure. Asshown in FIG. 3, the y-axis 302 corresponds to a viscosity at areservoir temperature and the x-axis 304 corresponds to polymerconcentrations of the oil recovery compositions. The target viscosity isillustrated by line 306. The lines 308, 310, and 312 illustrate theviscosity of an oil recovery composition at various polymerconcentrations and monovalent salt concentrations (that is, at variousratios of monovalent cations to divalent cations).

As further shown in FIG. 3, the addition of a monovalent salt, asillustrated by arrow 314, may increase the viscosity of an oil recoverycomposition to the target viscosity without requiring an increase inpolymer concentration. For example, as shown by lines 308, 310, and 312the target viscosity may be achieved by the addition of a monovalentsalt line at a lower polymer concentration than at higher polymerconcentration and lower monovalent salt concentration, as shown by line310 compared to line 308 and line 312 compared to line 310.

As mentioned supra, in some embodiments an enhanced oil recoverycomposition may be formed from produced water. FIG. 4 depicts a process400 for forming an enhanced oil recovery composition from a brinerecovered from produced water by modifying the monovalent cations todivalent cations ratio of the brine to achieve a target viscosity of thecomposition. It should be appreciated that the process 400 may besimilarly applied to brines obtained from other sources, such as treatedseawater, untreated seawater, aquifer water, etc. Initially, a brine maybe recovered from production water (block 400). The concentration ofmonovalent ions and divalent ions in the brine may be determined (block402), such as by determining the composition of the brine. Next, amonovalent salt may be added to the brine to modify the ratio ofmonovalent cations to divalent cations to a range of about 3:1 to about4.5:1 (block 404). For example, an amount of a monovalent salt such asNaCl may be added to the brine to form a modified brine, and thecomposition of the modified brine may be determined to evaluate themonovalent cations to divalent cations ratio. The brine and monovalentsalts may be mixed for a mixing time period (block 406) to ensure theelimination of salt precipitation and evaluate possible compatibilityissues that may affect suitability of the brine for injection.

Next, a polymer may be added to the brine and monovalent salt solutionto form the enhanced oil recovery composition having a monovalentcations to divalent cations ratio in a range of about 3:1 to about 4.5:1and having the target viscosity (block 408). The target viscosity may bedetermined for conditions of the hydrocarbon reservoir formation. Theconcentration of monovalent salt (or, in some embodiments, divalentsalt) added to achieve a target viscosity may depend on the selectedpolymer, the reservoir temperature, and the salinity of the brine. Insome embodiments, the long term heat degradation in the reservoir andthe shear degradation at constructions at the surface may also beconsiderations in achieving the target viscosity. In some embodiments,an achievable target viscosity may be limited by the injectivity of awell (that is, the reservoir characteristics and the associatedinjection volumes). In some embodiments, a surfactant may be added toform the enhanced oil recovery composition.

In some embodiments, the oil recovery an oil recovery composition havinga monovalent cations to divalent cations ratio in the range of about 3:1to about 4.5:1 may be used to enhance oil recovery from a hydrocarbonreservoir formation. The oil recovery composition may be prepared on thesurface by adding a monovalent salt to a brine, such as a brinerecovered from production water or from other sources (for example,treated seawater, untreated seawater, or aquifer water). For example, insome embodiments, an oil recovery composition having a brine and ahydrolyzable polymer, and having a monovalent cations to divalentcations ratio in the range of about 2.5:1 to about 3:1 may be preparedon the surface. In some embodiments, an oil recovery compositionprepared on the surface may be prepared to achieve a target viscosity.As discussed supra, preparation of the oil recovery composition may beperformed without the addition of water to the brine for dilation,without water treatment facilities, and without relatively highconcentrations of polymer. For example, a target viscosity of an oilrecovery composition having a hydrolyzable polymer may be achievedsolely by the addition of a monovalent salt without other additivesuntil a desired target viscosity (and corresponding monovalent cationsto divalent cations ratio) is reached. The oil recovery composition maybe introduced downhole (for example, injected or pumped) into thereservoir formation. In some embodiments, additional oil recoverycompositions or water may be subsequently introduced downhole. Displacedoil may then be recovered from the reservoir formation.

Ranges may be expressed in the disclosure as from about one particularvalue, to about another particular value, or both. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value, to the other particular value, or both, along withall combinations within said range.

Throughout this application, where patents or publications arereferenced, the disclosures of these references in their entireties areintended to be incorporated by reference into this application, in orderto more fully describe the state of the art to which the disclosurepertains, except when these references contradict the statements made inthe disclosure.

Further modifications and alternative embodiments of various aspects ofthe disclosure will be apparent to those skilled in the art in view ofthis description. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the embodiments described inthe disclosure. It is to be understood that the forms shown anddescribed in the disclosure are to be taken as examples of embodiments.Changes may be made in the elements described in the disclosure withoutdeparting from the spirit and scope of the disclosure as described inthe following claims. Headings used in the disclosure are fororganizational purposes only and are not meant to be used to limit thescope of the description.

What is claimed is:
 1. A method of forming a composition having a targetviscosity for enhancing oil recovery in a hydrocarbon reservoirformation: determining a mass ratio of monovalent cations to divalentcations of a brine; adding a monovalent salt to the brine to modify themass ratio of monovalent cations to divalent cations of the brine to arange of 2.5:1 to 3:1, the modified brine having a Na⁺ concentration inthe range of 22,166 milligrams/liter (mg/L) to 30.624 mg/L; and adding ahydrolyzable polymer to the brine to form the composition having thetarget viscosity, wherein the hydrolyzable polymer has a concentrationof 0.05 weight (wt) % to 0.5 wt %.
 2. The method of claim 1, the methodcomprising recovering the brine from a production water.
 3. The methodof claim 1, wherein the brine comprises treated seawater or untreatedseawater.
 4. The method of claim 1, wherein the brine comprises aquiferwater.
 5. The method of claim 1, comprising adding a surfactant to thebrine to form the composition.
 6. The method of claim 1, wherein thehydrolyzable polymer comprises polyacrylamide.
 7. The method of claim 1,wherein the monovalent cations comprise sodium cations.
 8. The method ofclaim 1, wherein the divalent cations comprise at least one of calciumcations and magnesium cations.
 9. The method of claim 1, wherein thetarget viscosity comprises at least 4 centipoise conditions of thehydrocarbon reservoir formation.
 10. The method of claim 1, whereinadding a monovalent salt to the brine comprises mixing the monovalentsalt and the brine for a mixing time period.
 11. The method of claim 1,wherein the monovalent salt comprises NaCl.